Stainless steel weld areas are typically difficult to inspect using ultrasound testing (UT) as relatively large anisotropic grains typically found in austenitic weld metals often distort and/or scatter the ultrasound beam. Most commonly, such deleterious effects are a combination of mode conversion and beam attenuation that is produced by variations in sound velocities amongst the grains with differing orientations and positions.
Mode conversion is an effect that is common in UT and often occurs when the ultrasonic beam strikes an interface between two materials with differing acoustic velocities at an oblique angle. When the beam impinges on the interface the beam is split into reflected and refracted beams having different modes and wave classifications (e.g., longitudinal, transverse, and surface waves). Mode conversions typically split the incident beam, reducing its strength, and produces undesired reflections that can create erroneous indications.
Furthermore, the anisotropic characteristics of stainless steels that produce mode conversion can also contribute to beam distortion, causing attenuation and scattering of the ultrasonic beam as it moves through the material. Attenuation generally refers to the absorption of the sound energy as it passes through the material to thereby generate heat. When the sound is absorbed, the signal-to-noise ratio is reduced making it difficult to distinguish the signal from the background noise. Signal scattering is the deflection of small amounts of acoustic energy out of the main ultrasonic beam. The deflection is the result of interactions between the sound beam and discontinuities in the material such as grain boundaries, inclusions, and defects (scattering is highly dependent on the relation between grain size and ultrasonic wavelength). Both attenuation and beam scattering are well recognized problems when using UT to inspect stainless steel weld areas.
At least some of the effects of mode conversion and beam distortion can be addressed and minimized by using suitable probes and analysis techniques. For example, it is known to reduce undesirable effects of attenuation by using lower frequency probes. However, use of lower frequency typically results in reduced sensitivity and resolution. Low signal-to-noise ratios due to scattering can be alleviated by using focusing probes. Unfortunately, where focused beams are used with standard twin crystal probes, inspection time will drastically increase as such processes often require numerous probes of differing angles and focal points.
Therefore, while some of the difficulties associated with UT of stainless steel may be overcome to at least some degree, all or almost all of such improvements require highly skilled technicians and/or significantly increased UT time. Thus, while numerous UT methods and devices are known in the art, all or almost all of them suffer from one or more disadvantages. Thus, there is still a need to provide improved ultrasound testing devices and methods.